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The present invention relates generally to a system for obtaining distance to target information in automated assembly and/or inspection machines.
Pick and place machines and solder paste inspection machines are common types of automated electronics processing machines. Pick and place machines are used in the surface mount technology (SMT) field to automatically place electronics components upon a circuit board during manufacture. Solder paste inspection machines are used to inspect small amounts of solder that are deposited upon a circuit board prior to placement of components thereon. Knowing the height between a workpiece, such as a circuit board, and the machine provides a number of advantages. First, in inspection machines, the Z-axis can be actuated to drive the an inspection camera towards or away from the workpiece so as to place the workpiece in the camera""s depth of field (i.e. focus).
Pick and place machines usually have a nozzle or quill that uses a vacuum to pick up a component, move the component to a desired location and lower the component onto the board. Knowing the height between the placement head and the board during placement provides the following advantages. First, if height is accurately known, the placement head can lower the component towards the board at a relatively high speed, then decelerate as the component is just above the board and gently place the component upon the board. Such placement facilitates high speed, accurate placement. If the height were not known, the placement head would have to decelerate to a low velocity much sooner than otherwise required in order to reduce the risk of jamming delicate components into the circuit board. Such early deceleration to a low velocity increases placement time and board cost. Thus, knowing the distance between the placement head and the circuit board prior to lowering the component allows each individual downward stroke to be optimized for speed and accuracy.
Another advantage provided by height knowledge relates to the accuracy of component placement. If the height is considered the Z-axis, then the placement apparatus can be thought to move along the z-axis as the component is lowered towards the circuit board. The circuit board theoretically defines an X-Y plane such that all X, Y and Z axes are orthogonal. In reality this is not the case. In fact, due to machine tolerance, build variations, and wear over time, the Z-axis is not perfectly perpendicular to the XY plane. This lack of perpendicularity causes errors in placement positions. This is illustrated by imagining that the placement machine drives the placement head exactly to the nominal position X,Y required to place a component at coordinate X,Y on the circuit board. Then the Z axis moves down a distance d to lower the part onto the circuit board. If the Z-axis is out of plumb by an angle p, the component will be placed with an error in the XY plane e=d*tan(p). While in some high accuracy placement machines it may prove difficult or costly to make the angle p sufficiently small for e to be negligible, it is relatively easy to measure p. If p is known, it is possible to compensate for e by having the placement head move to compensated coordinates Xxe2x80x2,Yxe2x80x2 before placement. The placement head""s accuracy is then improved to the extent that p is greater than zero and to the extent that d is unknown.
Another benefit of knowing the height would be to permit a less expensive non-telecentric fiducial finder lens to be used. In a non-telecentric system that has a certain depth of field, an object can be positioned over a range of distances, all within that depth of field and still be in focus. However, since the optic is non-telecentric, the apparent position of the object will have changed (as the object moves within that depth of field) unless, coincidentally, the object happened to be positioned at the center of the field of view. The apparent position change of the object represents a significant problem and drives the design of telecentric system for fiducial finding optics. Such systems have a depth of field, outside of which, an object will blur. However, the apparent position of the object will be substantially constant despite a change in distance. Telecentric optics are more expensive and bulky than non-telecentric optics. Therefore, a fiducial finding system that could use non-telecentric optics would be beneficial. With such a system, the height data could be used to post-process the image or the x-y result from the image to correct for non-telecentricity.
During the fabrication of electronic circuit boards, the boards are conveyed, or otherwise shuttled into a pick-and-place machine. Inside the machine, a circuit board is generally clamped and an assembly operation is performed. The assembly operation generally places components on the circuit board in their appropriate positions in preparation for permanent attachment. Typically, the circuit board is clamped in place, during the assembly operation, along two of the board""s edges that are in contact with the shuttle mechanism. After clamping, a support mechanism is generally raised from below the circuit board to support the center of the board. However, even after the board is clamped in place and the support is moved into position, the height of the board may vary from some nominal value. This is due, in part, to the fact that at least some boards while supported from below, may be bent up between the clamps, thus having an unknown height.
While the board under assembly may be bent to a relatively slight degree, advances in circuit board design and miniaturization have made the process ever more sensitive to such inconsistency. Specifically, the parts that must be placed on circuit boards are generally becoming smaller making them harder to handle and more susceptible to mechanical damage. Further, the smaller parts must generally be placed more accurately in order to ensure that they are properly coupled to the circuit board. Additionally, in order to minimize assembly time, and thus product cost, assembly machines are required to operate faster to provide enhanced throughput. Finally, the circuit boards themselves are becoming thinner and more flexible which further increases the variance of board heights.
A number of circuit board assembly operations are susceptible to variances in the board height. Such processes generally require knowledge of the board height and include solder paste inspection, component inspection, component part placement; and glue dispensation.
One method which could be employed to address the problem of varying board height is to actually measure the height between the nozzle and the board at the location under placement prior to the assembly operation. Several types of height measurement sensors are available that can provide such measurement including optical triangulation sensors. However, directly measuring the distance between the placement head and the circuit board directly under the nozzle is impossible for many assembly machines because surrounding parts already placed on the circuit board will generally occlude a clear line of sight at the placement or dispensing location. Further, the physical area around the placement nozzle is limited thus reducing the available choices for mounting locations and incidence angles of the height sensor.
Providing a system that could quickly acquire circuit board height information during placement would enhance circuit board assembly. Further, if a system were provided that required relatively little incremental hardware, adoption of such a system would be facilitated.
An automated system includes improved height sensing. In one aspect, an on-head camera performs the dual functions of fiducial imaging and height sensing using an auxiliary off-axis light source and triangulation. In another aspect, an on-head height sensor is positioned to measure height at a location that is not beneath any nozzles. The sensor provides height information at a plurality of locations over the board, and a height map of the board is created. In yet another aspect of the invention, the above features are combined to provide an on-head camera that images fiducials and measures height at a plurality of locations such that a height map is created.